1. Field of the Invention
The present invention relates to an image processing method and apparatus, and more particularly to a method of selecting color mapping parameters.
2. Description of the Related Art
Conventional color image processing apparatus includes apparatus in which an image displayed by an input system, such as a cathode-ray-tube (CRT) monitor attached to a host computing device, is reproduced by an output system such as an electrophotographic printer or ink-jet printer. Such apparatus typically includes a printer driver, running as software on the host computing device, that can display images on the input system, as well as having images printed by the output system.
In this type of apparatus, the input system displays color images by additive color mixing of three primary colors, normally red, green, and blue (RGB), while the output system prints the same color images by subtractive color mixing of three or four different colors, generally including cyan, magenta, and yellow (CMY) and often also including black (CMYK). One result is that the gamut of colors reproducible by the output system usually differs from the gamut of colors reproducible by input system. When color images are converted from the input system to the output system, the conversion is effected by a mathematical mapping of RGB data to CMY or CMYK data. Ideally, all output colors should exactly match the corresponding input colors, but because of the different gamuts of reproducible colors, this is not in general possible. Moreover, since different printers employ different types of toner or ink, the mapping parameters need to be adjusted individually for each output system in order to obtain at least a reasonably close match between the input and output colors.
The adjustment is often carried out by having the operator of the host computing device enter and adjust the numerical values of the coefficients of the mathematical mapping functions, or the values of what are termed color mapping vectors. It is not easy, however, to intuit the effect of a change in these numerical values on the colors printed by the output system. Even an experienced operator finds adjusting the color mapping parameters by this method a time-consuming process of trial and error, requiring much test printing and comparison of sample images. Accurate matching of colors between the input and output systems by this method is quite difficult.
Japanese Unexamined Patent Application Publication No. H09-217386 discloses an apparatus with slide-bars corresponding to the colors cyan, magenta, yellow, and black. The operator can change the mixing ratios of these colors in a stepwise manner by operating the slide-bars, instead of having to enter numerical parameter values. Manipulating slide-bars may be more intuitive than entering numerical values, but it is still difficult to visualize what the effect of a slide-bar adjustment will be on the output colors, so the operator must still proceed by trial and error, and accurate color matching remains difficult.